Earthquake

An earthquake is shaking of earth or vibrations on surface of earth.

What is Focus of Earthquake?

Point inside the surface of earth from where Earth Quake waves originate is called Focus of Earth Quake.

What is epicenter of Earthquake?

Point on the surface of earth above focus of earthquake or minimum distance from the focus of earthquake is called epicenter.

Why does an earthquake occur?

It occurs due to the release of elastic strain energy from the rocks which are under compression/tension.

During compression/tension, rocks accumulates elastic strain energy due to their elastic nature. Once this energy exceeds rock’s elastic limit, rocks are fractured and displaced and this elastic strain energy is released in the form of Seismic Waves.

How phenomenon of earthquake be explained through Plate tectonics?

(1) Convergent Plate Boundaries– These Plate Boundaries have greatest frequency and magnitude of earthquakes. At zone of Subduction, all three types of earthquake originates.

(a) Low Foci Earthquakes– The Foci of earthquake in not below 70 k.m. On Oceanic Side,  It is formed by initial bending and stretching of oceanic plate margin below the continental margin.

On Continental side, earthquake are generated by compression and over-thrusting of margins of continental plates.

(b) Intermediate Foci Earthquake– Their Foci is located between 70 k.m to 300 k.m below earth surface. earthquake is related with either compression of sub-ducting plate margins or its extension into mantle.

(c) Deep Foci Earthquake– Their Focus is below 300 k.m below earth surface. These are generated by compression forces of Mantle working on Sub-ducting Plate Margins.

Intermediate Foci and Deep Foci earthquakes are not destructive as most of their energy is lost before they reach to epicenter.

(2)- Divergent plate Boundaries– These earthquakes are related with tensional forces which are responsible for stretching and thinning of rocks which results in accumulation of elastic strain energy in the rocks. When this strain energy exceeds the elastic limits of the rock, rifting and drifting takes place and elastic strain energy is released in the form of seismic waves.

(3)- Transform Faults– During lateral movement, plates get locked together as these are hard and rigid segment. This locking of plates generates elastic strain energy, once it is above the force behind locking the plates there is sudden movement of plate with jerk and elastic strain energy is released in the form of seismic waves. These earthquake are high in magnitude and of low Foci.

What are seismic waves?

Seismic waves originate from the focus of earthquake, during an event of earthquake.

Seismic  waves are broadly classified into two categories.

1- Body Waves

2- Surface Waves

What are Body Waves?

Body waves are generated due to the release of energy at the focus and move in all the directions travelling through the body of the earth.

There are two types of body waves. They are P and S waves.

P WAVES S WAVES
1-      These are called Primary Waves. These are called Secondary Waves.
2-      Travel faster than S waves and are first to arrive at the surface. Travel slower than P Waves.
3-      They travel through Gaseous, liquid and Solid medium. They can travel only through Solid materials.
4-      They move parallel to the direction of the wave. This exert pressure on the material, as a result of which it creates density differences in the material leading to stretching and squeezing of the material. They move Perpendicular to the direction of wave. They create crests and troughs in the material through which they pass.

What are Surface Waves? 

The body waves interact with the surface rocks and generate a new set of waves called surface waves. These waves move along the surface. These waves are most Destructive. They cause displacement of rocks and hence, the collapse of structure( e.g Buildings, Bridges etc) occurs.

How Earthquakes are Measured?

The earthquake is measured according to the magnitude or intensity of the shock.

The magnitude scale is known as Richter Scale. The magnitude relates to the energy released during the quake. It is expressed in absolute numbers, Its range is 0-10. They are recorded by an instrument called Seismograph.

The intensity scale is called Mercalli Scale. It takes into account the visible damage caused by the event. The range of Intensity Scale is from 1-12.

What are harmful effects of an earthquake?

For details Click Here.

 

 

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